Fish eggs are far smaller relative to the adult size than is the case for most other aquatic organisms. This is consistent with theoretical predictions of competition as a result of density-dependent survival. According to this model, larger offspring may survive better in a competitive environment, but if the environment is less competitive it is more advantageous for parents to opt for higher fecundity.
In most aquatic animals, the size of the parent is a good predictor of the size of the offspring, but not so for the majority if the bony fish, whose eggs and larvae rarely measure more than a few mm. Our model suggests that competition among juveniles may be driving selection on offspring size. In this case, parents need to trade number for quality and either opt for few but large and competitive offspring, or many small larvae, few of which are likely to survive. The dual strategies predicted by the model are exemplified by the divergent offspring size/fecundity strategies found in most bony fish and in sharks, respectively.
See the article (Danish) in Aktuelt Naturvidenskab 5, November 2014 or at this link